1. Field of the Invention
The present invention relates to an information recording method for recording information in a recording medium having a recording layer which can reversibly achieve a crystal phase and an amorphous state by irradiating the recording layer with a multi-pulse laser light train emitted from a light source, and an optical recording medium for the information recording method.
2. Discussion of the Background
Along with the popularization of multimedia, playback-only optical recording media (hereinafter sometimes referred to as media) such as audio CDs and CD-ROMs and information reproduction apparatus have been developed and practically used. Recently, not only recordable optical discs using a dye medium and rewritable magnetooptic discs (MOs) using a magnetooptic medium, but also phase change type media attract attention.
Such phase change type media includes a material having a phase change property of reversibly achieving a crystal phase and an amorphous state and information is recorded therein utilizing the property. Information can be recorded in the phase change type media and the recorded information can be reproduced, using only laser light emitted from a laser diode (i.e., without using an external magnetic field), which is needed for recording information in MOs. In addition, it is possible to perform overwriting in which information can be recorded while erasing previously recorded information using laser light at the same time.
A typical waveform of recording laser light pulses for use in recording information in a phase change recording medium is the waveform of single pulse laser light emitted by a laser diode which is illustrated in FIG. 17 and which is generated according to, for example, an Eight Fourteen Modulation code (i.e., EFM). As can be understood from the waveform illustrated FIG. 17, the recording power PWA is set so as to be higher than the read power PR. When a pulse having such a waveform is applied to a phase change recording medium, problems such that the resultant recording mark deforms like a tear drop, and marks having a low reflectance against laser light cannot be obtained because the irradiated portion of the recording layer achieves an incomplete amorphous state due to slow cooling speed of the recorded mark.
In attempting to solve such problems, an information recording method which is illustrated in FIG. 18 is proposed for recording information in a phase change recording medium. In the method, as illustrated in FIG. 18, a mark is formed on a phase change recording medium by irradiating the recording medium with laser light which has a multi-pulse emission waveform and multilevel recording powers and which is generated according to an EFM code.
When a mark is recorded in a phase change recording medium using the multi-pulse laser light train, the corresponding portion of the multi-pulse laser light is constituted of a first heating pulse A by which the recording layer of the recording medium is preliminarily heated so as to be heated to a temperature not lower than the melting point thereof, plural heating pulses B which follow the first heating pulse A and by which the recording layer is further heated, plural cooling pulses which are located between the first heating pulse A and the top of the heating pulses B and between the heating pulses B, and a last cooling pulse C. In the method, the following relationship is satisfied:PWB PWA PWC≈PRwherein PWB represents the emission power of the heating pulse B, PWA represents the emission power of the first heating pulse A, PWC represents the emission power of the cooling pulse C and PR represents the read power.
When erasing a mark previously formed in a phase change recording medium using the multi-pulse laser light train, the corresponding portion of the multi-pulse laser light train is constituted of an erase pulse D. The emission power PED of the erase pulse D is set so as to satisfy the following relationship:PWC<PED<PWA.When this recording method is used, the mark area of the recording layer achieves an amorphous state because the heated area is rapidly cooled, and the space area achieves a crystal state because the area is heated and then gradually cooled without forcible cooling. Thus, the recording medium has a large reflectance difference between the mark (i.e., the area in an amorphous state) and space (i.e., the area in a crystal state).
The methods for recording information in an optical recording media are classified into a mark position recording method (i.e., Pulse Position Modulation, PPM) and a mark edge recording method (i.e., Pulse Width Modulation, PWM). Recently, the mark edge recording method is typically used because of being able to be used for high density recording. When information is recorded in a phase change recording medium by a mark edge recording method, the heating pulse typically has a pulse width of 0.5T and the cooling pulse also has a pulse width of 0.5T, wherein T represents a record channel clock cycle.
Namely, information is recorded in a phase change recording medium using laser light having a multi-pulse emission waveform in which a pair of a heating pulse and a cooling pulse is added whenever the mark length (i.e., mark data length) of recording data increases by 1T. FIG. 19 illustrates a typical example of the recording waveform. When high speed recording is performed using this method, the record channel clock (T) is highly frequented at the same rate as that of the linear recording speed, for example, at a rate of twice or four times, without changing the record waveform.
However, as the recording speed increases, the width of the heating pulse and cooling pulse seriously decreases, and thereby it becomes impossible to heat the recording layer so as to reach the heating temperature (i.e., the amorphous temperature) and the cooling temperature (i.e., the crystal temperature), at which the recording layer can change the phase, resulting in formation of incomplete marks. Thus, a problem in that the mark does not have a predetermined mark length occurs.
In attempting to solve the problem, Unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 9-134525 discloses an information recording method in which a recording mark having a desired mark length is recorded in a recording layer at a high speed by heating and cooling the recording layer for a time enough to sufficiently heat and cool the recording layer without driving a light source driver at a high speed. Specifically the recording method is such that a light source irradiates a phase change recording layer with a first heating pulse followed by plural rear heating pulses, plural rear cooling pulses which are emitted between the first heating pulse and the top of the rear heating pulses and between the plural rear heating pulses, and a last cooling pulse to record a mark therein, wherein when data having a mark length of nT (n is odd or even numbers, and T represents a record channel clock cycle) are recorded, the pulse width of the rear heating pulses and rear cooling pulses is substantially the same as the record channel clock cycle.
In addition, an optical recording medium which has a GeSbTe recording layer and in which information can be recorded in the recording layer at a speed 4.8 times the recording speed of DVD using the information recording method disclosed in JP-A 9-134525 is disclosed in Optical Data Storage (ODS) 2000 Technical Digest (pp. 135–143).
However, when information is repeatedly recorded in a melt-erase mode in the optical recording medium disclosed in ODS 2000 Technical Digest at a speed 4.8 times the DVD speed (i.e., at a speed of 16.8 m/s) using the information recording method disclosed in JP-A 9–134525, the space area of the recording medium does not perfectly achieve a crystallization state (i.e., a part of the space area maintains the amorphous state) because the crystallization speed of the recording medium is not fast enough to match the recording speed, i.e., the recording medium has poor repeat recording properties.
In attempting to solve the problem (i.e., to avoid amorphism of an space area), ODS 2000 Technical Digest discloses a method in which recording is repeatedly performed while the erasing power Pe is decreased to a power such that the recording layer is not fused. However, the recording properties of the information recorded by the method are inferior to those of the information recorded in a melt-erase mode.